Microporous polyolefin membranes are widely used in separators for lithium batteries, etc., electrolytic capacitor separators, various filters, steam-permeable, waterproof clothing, etc. When the microporous polyolefin membranes are used as battery separators, their performance largely affects the performance, productivity and safety of batteries. Accordingly, microporous polyolefin membranes are required to have excellent permeability, mechanical properties, heat shrinkage resistance, shutdown properties, meltdown properties, etc.
In general, microporous membranes made only of polyethylene have low meltdown temperatures, while microporous membranes made only of polypropylene have high shutdown temperatures. Accordingly, microporous membranes based on polyethylene and polypropylene are suitable for battery separators. Thus proposed are microporous membranes made of a mixture of polyethylene and polypropylene, and multi-layer, microporous membranes comprising a polyethylene layer and a polypropylene layer.
For instance, JP 05-251069 A and JP 05-251070 A disclose separators free from thermal runaway, which are formed by a multi-layer, microporous sheet comprising a first layer made of an ethylene-butene copolymer or an ethylene-hexene copolymer, which undergoes shutdown at 80 to 150° C., and a second layer made of polypropylene, which undergoes shutdown at a higher temperature than in the first layer by 10° C. or more.
JP 05-251069 A discloses a method for producing a multi-layer, microporous sheet comprising the steps of producing a laminate sheet comprising a layer made of the above copolymer and an extractable solvent and a layer made of polypropylene and an extractable solvent, removing the extractable solvent from the sheet to obtain a microporous sheet, and stretching the microporous sheet at a temperature of 25 to 110° C. JP 05-251070 A discloses a method for producing a multi-layer, microporous sheet comprising the steps of simultaneously extruding a melt of the above copolymer and a polypropylene melt, cooling the melts to obtain a laminate sheet, stretching the laminate sheet at a temperature from −198° C. to −70° C., and heat-treating the laminate sheet.
However, investigation by the inventors have revealed that the methods described in the above references produce multi-layer, microporous membranes comprising a polypropylene layer and a polyethylene layer, which have small pore diameters in the polypropylene layer, thereby having insufficient permeability.
JP 62-10857 A proposes a battery separator having excellent dimensional stability and shutdown properties, which comprises (a) a first layer formed by a microporous sheet of polyethylene or polypropylene, which loses pores with its size substantially unchanged at a temperature of about 80° C. to 150° C., and (b) a second layer formed by a microporous sheet made of polyethylene or polypropylene and filler particles, which keeps its microporous structure and size in a range from room temperature to a temperature higher than the pore-losing temperature of the first layer by at least about 10° C. However, the battery separator of JP 62-10857 A does not have sufficient mechanical strength, because the filler particles do not have optimized aspect ratios.
Thus desired is a multi-layer, microporous polyolefin membrane comprising a polyethylene-containing layer and a polypropylene-containing layer, thereby having well-balanced permeability, mechanical strength, heat shrinkage resistance, shutdown properties and meltdown properties.